1. Field of the Invention
This invention relates to an electroluminescent device of compound semiconductor. More particularly, it is concerned with an electroluminescent device of compound semiconductor in which a compound semiconductor selected from zinc sulfide (ZnS), zinc selenide (ZnSe) and a mixed crystal thereof is used.
2. Description of the Prior Art
ZnS and ZnSe are known as a material for a blue light-emitting diode and other various electroluminescent devices of producing lights falling within the range of from ultraviolet rays to all visible rays. FIG. 4 and FIG. 5 each shows a basic construction of the conventional blue light-emitting diode manufactured from the material. Precisely, FIG. 4 is a cross sectional view of a metal-insulator-semiconductor (MIS) type electroluminescent device using ZnS as a light-emitting layer, where 31 is a low-resistivity n-type ZnS substrate, 32 is a ZnS insulating layer for hole injection, which is made of a high-resistivity ZnS, and 33 and 34 each is a metal electrode as formed on the low-resistance n-type ZnS substrate 31 and one as formed on the ZnS insulation layer 32, respectively. The substrate 31 is an n-type ZnS single crystal substrate having an resistivity of from 1 to 10 .OMEGA.cm, which is prepared by heat-treating a ZnS bulk single crystal as grown by an iodine transport method using iodine as a transporting medium, in a molten zinc of 1000.degree. C. for 100 hours or more so as to lower the resistivity of the resulting crystal substrate.
The ZnS insulation layer 32 is epitaxially grown on the ZnS substrate 31 by organometallic vapor phase epitaxy (OMVPE); Au is plated over the ZnS insulation layer 32 by vacuum deposition to form the positive electrode 34; and an In-Hg amalgam is coated on the back surface of the n-type ZnS substrate 31 followed by heat-treatment in a pure hydrogen for a period of from several tens of seconds to several minutes to form the ohmic negative electrode 33. Accordingly, the conventional ZnS MIS-type blue light-emitting device is constructed (K. Hirahara et al., Extended Abstracts of the 15th conf. on SSDM, Tokyo, 1983).
FIG. 5 is a cross sectional view of a p-n junction-type blue light-emitting device of ZnSe, in which 41 is a low-resistance p-type GaAs substrate, 42 is a light-emitting layer made of a p-type ZnSe, 43 is a light-emitting layer made of an n-type ZnSe, and 44 and 45 each is a metal electrode as formed on the p-type GaAS substrate 41 and one as formed on the n-type ZnSe light-emitting layer 43. The two epitaxial layers 42 and 43 as formed on the GaAs substrate 41 are formed by OMVPE like the case of the above-mentioned ZnS MIS-type electroluminescent device. As electrodes for applying voltage, a negative electrode 45 made of In is formed on the n-type ZnSe light-emitting layer 43 and a positive electrode 44 made of In is on the back surface of the p-type GaAs substrate 41. Accordingly, the conventional ZnSe p-n junction-type blue light-emitting electroluminescent device is constructed (T. Yasuda et al., "Metalorganic Vapor Phase Epitaxy of Low-Resistivity p-Type ZnSe", Appl. Phys. Lett., 52 (1988), pp. 57 to 58; K. Akimoto et al., "Electroluminescnece in an Oxygen-Doped ZeSe p-n Junction grown by Molecular Beam Epitaxy", Japanese Journal of Akimoto et al., "Electroluminescence from a ZnSe p-n Junction Fabricated by Nitrogen-Ion Implantation", ditto, pp. L528 to 530 ).
In the above-mentioned ZnS MIS-type blue light-emitting electroluminescent device (FIG. 4), a tabular bulk single crystal is used as the light-emitting layer since a high-quality ZnS epitaxial film is difficult to obtain. In such constitution, however, it is extremely difficult to control the light-emitting characteristic by lowering the resistivity of the layer by the heat-treatment. Moreover, as the device has MIS constitution, the current-injecting efficiency is low and, as a result, it is extremely difficult to attain light-emission with high efficiency and high luminance. Furthermore, there is an additional problem that the light-emitting spectrum is broad with an inferior monochromaticity, since the light-emission system of the device is one for giving a blue color by emission from a deep level.
On the other hand, in the p-n junction-type electro-luminescent device of FIG. 5, light-emission near the band edge is utilized for emitting blue light. Therefore, the light-emitting spectrum is sharp with an excellent monochromaticity. Moreover, as the device has p-n junction structure, the current-injection efficiency may be elevated. However, as the device uses a III-V compound semiconductor substrate such as GaAs or GaP substrate or uses an Si substrate, it is difficult to obtain a ZnSe epitaxial layer having a crystallinity necessary as a light-emitting material because of the lattice mismatch between ZnSe forming the light-emitting layer and the substrate and because of the mismatch of the thermal expansion coefficient therebetween. Moreover, since the substrate-constituting elements would diffuse into the ZnSe epitaxial film by thermal diffusion, it is not easy to control the impurities in the light-emitting layer. Therefore, there is a severe problem that formation of a high-quality ZnSe light-emitting layer necessary for obtaining a high-efficiency light-emission is extremely difficult. Additionally, there is another problem in that elevation of the efficiency of taking out the emitted blue light is difficult since all of GaAs, GaP and Si to be employed as the substrate material have a property of absorbing the blue light as emitted from the light-emitting layer.